Using freed memory

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Abstract

Referencing memory after it has been freed can cause a program to crash.

Overview

The use of heap allocated memory after it has been freed or deleted leads to undefined system behavior and, in many cases, to a write-what-where condition.

Use after free errors occur when a program continues to use a pointer after it has been freed. Like double free errors and memory leaks, use after free errors have two common and sometimes overlapping causes:

• Error conditions and other exceptional circumstances
• Confusion over which part of the program is responsible for freeing the memory

Use after free errors sometimes have no effect and other times cause a program to crash. While it is technically feasible for the freed memory to be re-allocated and for an attacker to use this reallocation to launch a buffer overflow attack, we are unaware of any exploits based on this type of attack.

Consequences

• Integrity: The use of previously freed memory may corrupt valid data, if the memory area in question has been allocated and used properly elsewhere.

• Availability: If chunk consolidation occur after the use of previously freed data, the process may crash when invalid data is used as chunk information.

• Access Control (instruction processing): If malicious data is entered before chunk consolidation can take place, it may be possible to take advantage of a write-what-where primitive to execute arbitrary code.

Exposure period

• Implementation: Use of previously freed memory errors occur largely at implementation time.

Platform

• Languages: C, C++, Assembly

• Operating Platforms: All

Required resources

Any

Severity

Very High

Likelihood of exploit

High

Avoidance and mitigation

• Implementation: Ensuring that all pointers are set to NULL, once the memory they point to has been freed, can be effective strategy. The utilization of multiple or complex data structures may lower the usefulness of this strategy.

Discussion

The use of previously freed memory can have any number of adverse consequences - ranging from the corruption of valid data to the execution of arbitrary code, depending on the instantiation and timing of the flaw.

The simplest way data corruption may occur involves the system's reuse of the freed memory. In this scenario, the memory in question is allocated to another pointer validly at some point after it has been freed. The original pointer to the freed memory is used again and points to somewhere within the new allocation. As the data is changed, it corrupts the validly used memory; this induces undefined behavior in the process.

If the newly allocated data chances to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.

Examples

Example1:

#include <stdio.h>
#include <unistd.h>

#define BUFSIZER1   512
#define BUFSIZER2   ((BUFSIZER1/2) - 8)

int main(int argc, char **argv) {   
    char *buf1R1;
    char *buf2R1;
    char *buf2R2;
    char *buf3R2;

    buf1R1 = (char *) malloc(BUFSIZER1);
    buf2R1 = (char *) malloc(BUFSIZER1);

    free(buf2R1);

    buf2R2 = (char *) malloc(BUFSIZER2);
    buf3R2 = (char *) malloc(BUFSIZER2);

    strncpy(buf2R1, argv[1], BUFSIZER1-1);
    free(buf1R1);
    free(buf2R2);
    free(buf3R2);
}

Example2:

	char* ptr = (char*)malloc (SIZE);
	...
	if (err) {
		abrt = 1;
		free(ptr);
	}
	...
	if (abrt) {
		logError("operation aborted before commit", ptr);
	}

Related problems

• Buffer overflow (in particular, heap overflows): The method of exploitation is often the same, as both constitute the unauthorized writing to heap memory.

• Write-what-where condition: The use of previously freed memory can result in a write-what-where in several ways.